Dyestuff compositions are widely employed to color natural and synthetic fibers. Such compositions generally contain, in addition to the dyestuff component, a dispersant for the dyestuff. In the dyestuff composition, the dispersant generally serves three basic functions:
(1) it assists in reducing the dye particles to a fine size in a grinding operation; PA1 (2) it maintains the dispersing medium for the dyestuff; and PA1 (3) is serves as a diluent.
Sulfonated compounds are commonly employed as dye dispersants and may be obtained from two major sources, sulfonated lignins from the wood-pulping industry prepared from the lignin by-products of a sulfite or a kraft pulping process, and naphthalene sulfonates derived as by-products from the petroleum industry. The advantages of employing sulfonated lignins as dispersants in dyestuff compositions are due to their unique physical properties which include good capability to many dye systems, outstanding dispersant characteristics at ambient and elevated temperatures, and availability. There are, however, certain disadvantages in employing such lignins, whether sulfite process-derived lignins, or sulfonated kraft lignins, as dipersants. Negative factors in the use of such lignins as dyestuff additives have in the past related to problems of foaming, high pH, fiber staining, poor heat stability, and the tendency of sodium and ammonium salts of the sulfonated lignins to cause some color reduction of the sensitive azo dyestuffs employed in the dyeing industry.
Technological developments have resulted in methods and processes to modify sulfonated lignins to reduce many of the negative aspects of their use as dispersants, without adversely affecting their desirability as a dyestuff dispersant. The following patents are exemplary of modifications and processes employed in the production of lignin materials to improve their use in dyestuff systems: U.S. Pat. Nos. 4,001,202 to P. Dilling et al.; 4,184,845 to S. Y. Lin; 4,131,564 to P. Dilling; 3,156,520 to L. A. Blaisdell; 3,094,515 to K. F. Keristead et al; 3,726,850 to Detroit; 2,680,113 to E. Adler et al.; 3,769,272 to Hintz; 3,841,887 to Falkehag et al.; 4,355,996 to P. Dilling et al; 4,308,203 to Lin; 4,454,066 to Dilling et al.; 4,534,771 to Dilling; and 4,546,173 to Dilling.
Because of their availability, lignin by-products of a kraft wood-pulping process in the paper-making industry are widely employed as dyestuff dispersants, and a commonly used form of dye dispersant is the sodium salt of a sulfonated lignin. The recovery, chemical modification, and use of such sulfonated lignin sodium salts are described in commonly assigned co-pending U.S. patent application Ser. No. 06/657,973 U.S. Pat. No. 4,590,262 and Ser. No. 06/819,600.
Lignin is commonly obtained from the spent pulping liquors of a pulping process in the paper-making industry, wherein lignocellulosic materials, such as wood, straw, corn stalks, bagasse, and the like, are processed to separate the cellulosic pulp from the lignin. In a sulfite pulping process, the lignin in the wood undergoes hyrolysis and in situ sulfonate allowing it to become soluble. A variety of commercial sulfite processes exist today. Among these are the magnesium-based pulping process, the calcium-based process, the ammonia-based process, the soda, the neutral, the bisulfite, and acid sulfite processes. In a kraft pulping process, lignin is obtained from the pulping liquor, known as black liquor, by subjecting the same to the effects of strong alkali, such as sodium hydroxide, to solubilize the lignin component in its sodium salt form. The sodium salt form of the lignin can then be separated from the cellulose and recovered from the pulping liquor by acidification. Acidification of the black liquor, such as by introduction of carbon dioxide, from a pH of around 13 to a pH of about 9.5, converts the phenolic hydroxide groups on the lignin molecule from their ionized form to their free phenolic, or acidic, form in which they precipitate. The precipitated lignin may be further acidified by the addition of acid, such as sulfuric acid, to a low pH, where it is water washed to remove undeired inorganic salts and other impurities.
In order to enhance the solubility of the lignin over a larger pH range, and thus facilitate its use as a dye dispersant, a kraft process lignin may be sulfonated by reaction with a sulfur-containing compound to a variety of specific degrees. Lignin salts obtained from a kraft, soda, or other alkaline process are not recovered as a sulfonted product, but may be sulfonated by reacting the lignin with suitable materials. Kraft process lignins which are sulfonated by attachment of sulfonic acid groups to the side chain of the lignin are commercially available and certain of them are sold under the trademark POLYFON.RTM. by Westvaco Corporation of Charleston, S. C.
Kraft process lignins may also be sulfonated by attachment of sulfonic acid groups on the aromatic nuclei of the phenolic group of the lignin by a methylolation and sulfonation reaction. Such sulfomethylated lignins are commercially available and produced under the trademark REAX.RTM. by Westvaco Corporation. Kraft process lignins recovered from black liquor also contain catechol components in the lignin molecule which also react with components of a sulfonation or a sulfomethylation reaction.
Sulfomethylation may be carried out by simultaneous reaction of an aldehyde and a sulfur-containing compound with the lignin, or sequentially after the aldehyde methylolation step and water wash, by addition of a bisulfite or sulfite compound to provide an effective amount of sulfonate groups to give the desired water-solubility in moderately acid and higher pH solutions. The degree of sulfonation of a lignin is proportional to its solubility in an aqueous solution and the viscosity of the lignin. Sulfomethylation of lignin by-products are disclosed in U.S. Pat. No. 2,680,113 to Adler et al.
Recently, it has also been proposed to produce an ammonium salt of a sulfomethylated lignin from the lignin by-product of a kraft pulping process, as well as an amine salt of such a sulfomethylated lignin, for use as dyestuff additives and dispersants. Such products and their methods of manufacture are disclosed in commonly assigned co-pending U.S. patent application Ser. No. 06/679,901 now U.S. Pat. No. 4,642,336 and Ser. No. 06/783,781, respectively.
As aforementioned, although sodium salts of sulfonated lignins are widely employed and are effective in many applications as a dyestuff dispersant, they have a tendency to reduce the color intensity of dyestuff systems employing sensitive azo dyestuffs. Ammonium salts of sulfomethylated lignin also cause color reduction in azo dyestuff-containing dye systems, although to a lesser extent.